Question Potentiometer purpose and explanation

Hi all,

I have started my build thread in the projects area (https://www.xsimulator.net/community/threads/first-diy-2dof-attempt.10333/) and I am getting together the physical components but I have come across something I don't fully understand. I am planning on doing the basic Arduino/MM/Wiper motor build. I think i understand enough that they provide feedback/positional information BUT two questions:

1. Does it matter what "K"/OHM value the potentiometers are?
2. What is the deal with "gearing" the potentiometers?​

Some projects i have seen, the potentiometers are geared one to one to the main motor shaft, some have a bigger gear than the motor shaft and some have a smaller gear than the motor shaft.

I am happy to read, learn and try to understand more so I have more of an idea of what I am doing but I am just not sure where I need to be reading if this type of question has been asked before.

Thanks in advance

1. The Arduino has 0 to 5V analogue inputs which are 10 bit, therefore it's resolution is...
0-5V input range = 0-1023 counts = 4.88mV resolution
This means that the smallest detectable movement ideally wants to produce approx 5mV.
2. The pots are geared so that the rotation of the motor or movement of the actuator, is geared to produce a full 0 to 5V signal to the analogue input.

I would hate to think what you dream of at night @noorbeast, you have all the right threads for the rights questions...

About #1 - It does matter what the resistance of the potentiometers are. I didn't see it right off but since you are running with Monster Motos, I'm guessing you are using Arduino Uno's to control them and that is what would be reading the potentiometers.

If you check here: https://www.arduino.cc/en/Tutorial/AnalogInput, Arduino recommends a 10k pot for this kind of thing. I'd stick with that. You can go up or down from that, but at low resistance you will use more current in the pot which depending on the value can lead to heating and drift, and at too high of resistance, the input resistance of the Atmel chip ADC can load the pot and cause slow and nonlinear response. Atmel also recommends a 10k ohm or less output impedance for whatever the circuitry is you are measuring.

But I agree with Noorbeast - Hall sensors are the way to go. There are versions that look just like pots with an input shaft you have to use flex couplings to keep from wearing out, and there are also totally non-contact versions that you mount a magnet holder on your motor shaft and mount the sensor body in proper position next to it. No flex coupling, no wear, and no wearing out. I use Honeywell RTP180LVEAA sensors. A bit more than what some recommend, but so easy to install, rugged, and again, no wear. But there are others too.

Simple question, is ok to use 20k potentiometers? I can get some really good 20k pots cheap...

Hi Mariano, 20k pots might work but they are not the recommended value for the Arduino Uno. You don't say what will be measuring the voltage on the pot, so that may or may not be correct for you.

In a typical setup and without gearing, you would only see symmetric charge and discharge of the input ADC sample and hold capacitor of the Atmel (assuming Atmel based control circuit) at mid travel on the pot. As you move to either end, one way the charge impedance would be fine but the discharge would be too high. To the other end that would swap. Chances are you would be ok but you want to be sure to aim for mid range on the pot at mid range on your actuator to have the best performance each way. Thing is that the time scale of rig motion is way slower than the time scale of the ADC Sample and hold cap charging and discharging. You could be fine just because of that.

Pots are cheap, though. I'd try for 10k if you can though the 20k's may work fine in practice. Only way to know is to try. Maybe the errors won't be noticeable.

Thank you Zed for the lesson, now the 50k are out of stock and 5k are in stock at half the price (us40 instead of us80 for 4 of them) elusive 10k pots

In most setups the pots just act like variable voltage dividers with the voltage over the whole resistive element. If you need inputs in the range of 0-5V then you would probably be using 5V and ground at the ends. With a 5k ohm element, that's just 1 mA of current through the element. Power dissipated would only be 5 mW. Probably very little heating and no problem with drift.